A single batch synthesis of pure phase Mo2C from ammonium molybdate: pathway and properties

TL;DR

This paper introduces a new method to make pure molybdenum carbide (Mo2C) from ammonium molybdate without producing carbon dioxide, using controlled gas atmospheres and temperature changes.

Contribution

A novel CO2-free synthesis pathway for phase-pure Mo2C from ammonium molybdate using sequential gas atmosphere changes.

Findings

57.67% of AMT was converted to Mo2C, matching the theoretical value of 57.74%.

XRD confirmed phase-pure Mo2C with distinct diffraction peaks at 34.176°, 37.712°, and 39.197°.

SEM showed fine Mo2C particles (0.1 μm) formed from coarse AMT particles (>50 μm).

Abstract

This study presents an original, effective, and environmentally friendly method for synthesizing pure molybdenum carbide (Mo2C) from ammonium molybdate tetrahydrate (AMT) without generating carbon dioxide, a greenhouse gas. The process involves the sequential transformation of AMT to Mo2C, which follows the reaction pathway of (NH4)6Mo7O24→MoO3→MoO2→Mo→Mo2C. This transformation is achieved by strategically altering the gas atmosphere, switching from Ar to H2 at 800 K and then from H2 to CH4 at 1000 K. Thermal analysis, X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques were used to characterize AMT and the products. Mass measurements were used to follow the conversion of AMT to intermediate products and to the final product (Mo2C). It was found that 57.67% of AMT was converted to Mo2C, in agreement with the theoretical value (57.74%). Differential scanning calorimetry/thermogravimetry curves revealed four steps at 401 K, 495 K, 507 K, and 595 K during AMT decomposition to MoO3. XRD patterns revealed the formation of phase-pure Mo2C, with characteristic diffraction peaks 2θ = 34.176°, 2θ = 37.712°, and 2θ = 39.197° assigned to the (100), (002), and (101) crystal planes, respectively. SEM images showed that fine Mo2C particles with a thickness of 0.1 μm was obtained from very coarse AMT particles (>50 μm). In order to determine the solid and gaseous phases likely to form during the reaction, thermodynamic analysis using Gibbs’ free energy minimization method was also carried out prior to synthesis. The reduction reactions and the resulting morphologies of the synthesized materials were discussed in terms of thermodynamic results and density changes associated with the conversions. This study demonstrates a novel reaction pathway that sequentially converts the molybdenum species from Ammonium Molybdate Tetrahydrate (AMT) to the final Mo2C phase without the release of CO2.

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